1. Create a new blank iOS project
2. Create a new Storyboard / UIViewController
3. Open info.plist to set the startup interface
4. Select the desired item from the list
Run the project
The selected item isn't the main interface, in fact, there is a crash.
Go back to the info.plist
Notice that there isn't an item selected.
The bug seems to be that if there isn't a key for UIMainStoryboardFile then it will not be created modified.
If the key is added manually then everything works, and it can be updated.
It also might be a good idea to have QA test the project templates for oddities.
This bug is already fixed in our 15.4 preview 3 release.
Thanks for reporting!